E-mail systems are ubiquitous in the landscape of today's communication infrastructures. Unfortunately as e-mail communications have become more prevalent so too have unsolicited mass e-mailings, commonly referred to as junk e-mail. Typically a junk e-mail sender will send hundreds of thousands of e-mails to random e-mail addresses in the hope that these unsolicited messages will be opened and read by the unwitting recipients.
Increasingly e-mail systems have incorporated functionality that allows messages sent in HTML to be rendered and viewed within the e-mail application. Sending e-mail messages as HTML is particularly attractive as it allows for the body of the e-mail to be displayed as it would appear in a web browser, thus allowing for the incorporation of, for example, externally hosted image files and navigable hyperlinks.
Typically, when an e-mail client application renders the HTML page it automatically downloads pictures and other “inline” content referenced within the HTML page at the time of rendering. For example, an e-mail message sent as HTML contains a number of HTML defined “elements”, such as <A> (anchor tag) or IMG (image tag). HTML elements have “attributes” which may be used to modify the interpretation of the element. HTML elements and attributes are used by the web browser (i.e., e-mail client application) to correctly format and render the HTML page. One particular type of element that the HTML may contain is an image tag which indicates that an image is to be displayed within the rendered page. The image can be specified in the tag as being hosted on an external server which can be referenced through the Internet. As the e-mail client renders the HTML in the e-mail message it will process the various HTML elements at the time the page is rendered. Thus, when the e-mail client renders the page, the image specified, and other external content specified, by the image tag will be obtained from the external server so that it may be included in the HTML page as displayed. Other examples of elements and attributes which automatically obtain external content are BACKGROUND, BGSOUND, and stylesheet LINK.
While the ability to send an e-mail message with an HTML body and to have the message rendered in an e-mail client is beneficial, junk e-mail senders have unfortunately seized on the inherent features of this functionality to develop a technique which enables them to detect “live” e-mail addresses and opened/read e-mail. Through the use of a “web beacon”—a link in the HTML body of an e-mail message that is automatically dereferenced by an e-mail client program while rendering the body and that contains unique information allowing for identification of the message recipient and the instance or version of the message itself—junk e-mail senders can detect that the recipient has actually opened the message. This information allows junk e-mail senders to validate the validity of recipient's e-mail address, effectively behaving as the equivalent of an e-mail read receipt, and thus collect statistical information without any control or knowledge by the person who is the junk e-mail target (while e-mail has the concept of a read receipt the recipient has the ability to suppress the sending of the response whereas this mechanism circumvents such an ability).
In addition when an e-mail message sent as HTML is rendered by the e-mail client (i.e., a server-generated HTML page), referenced external images and activated links will cause most browsers to include an HTTP Referer header. This optional header field allows the client to specify, for the server's benefit, the address (URL) of the document (or element within the document) from which the URL in the request was obtained. In case of a web-based e-mail client, the original document is basically the message in the client's mailbox on the server, hence its URL usually contains the server name and mailbox name, as well as the message subject as the resource name. Thus, it is possible for a moderately astute junk e-mail sender to send mail with links and images linking to their site in an effort to comb their web logs and determine a user's server name, a user's e-mail address, and the subject of the mail in which the user clicked on a link.